Installation and process for the preparation of urea

ABSTRACT

The invention relates to an installation for the preparation of urea from ammonia and carbon dioxide, the installation comprising two reactor sections in a vertically placed combined reactor and a high-pressure condenser section. The installation may comprise a vertically placed combined reactor, with the two reactor sections being separated by a high-pressure condenser section. In another embodiment the installation comprises a vertically placed combined reactor that comprises two reactor sections and a high-pressure condenser section placed outside the reactor. The invention also relates to a process for the preparation of urea in this installation. This involves feeding the gas stream leaving the stripper wholly or partly to the high-pressure condenser section of the installation. Preferably, a portion of the gas stream leaving the scrubber is fed to the second reactor section in the vertically placed combined reactor via an ammonia-driven ejector.

[0001] The invention relates to an installation for the preparation ofurea. The invention also relates to a process for the preparation ofurea in this installation.

[0002] Urea can be prepared by introducing an ammonia excess togetherwith carbon dioxide at a pressure between 12 and 40 MPa and at atemperature between 150 and 250° C. into a urea synthesis zone. Theresulting urea formation can be represented best in the form of twoconsecutive reaction steps, in the first step ammonium carbamate beingformed according to the exothermic reaction:

2NH₃+CO₂→H₂N—CO—ONH₄

[0003] after which the ammonium carbamate formed is dehydrated in thesecond step to give urea according to the endothermic equilibriumreaction:

H₂N—CO—ONH₄⇄H₂N—CO—NH₂+H₂O

[0004] The extent to which these reactions take place depends amongstother things on the temperature and the ammonia excess used. Thereaction product obtained is a urea synthesis solution substantiallyconsisting of urea, water, unbound ammonia and ammonium carbamate. Theammonium carbamate and the ammonia are removed from the solution and arepreferably returned to the urea synthesis zone. In addition to theabove-mentioned solution in the urea synthesis zone a gas mixture isformed which consists of unconverted ammonia and carbon dioxide togetherwith inert gases, the so-called reactor off-gas. Ammonia and carbondioxide are removed from this gas mixture and are preferably alsoreturned to the urea synthesis zone. The urea synthesis zone maycomprise separate zones for the formation of ammonium carbamate andurea. These zones may, however, also be combined in a single apparatus.

[0005] In practice, various processes are used for the preparation ofurea. At first, urea was prepared in so-called conventionalhigh-pressure urea plants. At the end of the 1960s, however, thisprocess was succeeded by processes carried out in so-called ureastripping plants.

[0006] The conventional high-pressure urea plants that are currentlystill operating are understood to be urea plants in which thedecomposition of the ammonium carbamate not converted into urea and theexpulsion of the usual ammonia excess take place at a substantiallylower pressure than the pressure in the synthesis reactor itself. In aconventional high-pressure urea plant the synthesis reactor is usuallyoperated at a temperature of 180-250° C. and a pressure of 15-40 MPa.Furthermore, in a conventional high-pressure urea plant ammonia andcarbon dioxide are fed directly to the urea reactor. In a conventionalhigh-pressure urea process the molar NH₃/CO₂ ratio (=N/C ratio) in theurea synthesis zone lies between 3 and 6. Depending on the extent towhich the unconverted ammonia and carbon dioxide are returned to theurea synthesis section in conventional urea plants, a distinction ismade between Once Through (no recycle), Partial Recycle (only partialrecycle of ammonia and/or carbon dioxide) or Total Recycle (both ammoniaand carbon dioxide recycle) plants.

[0007] A urea stripping plant is understood to be a urea plant in whichthe decomposition of the ammonium carbamate that has not been convertedinto urea and the expulsion of the usual ammonia excess largely takeplace at a pressure that is essentially almost equal to the pressure inthe synthesis reactor. This decomposition and expulsion take place inone or more stripper(s) installed downstream of the synthesis reactor,preferably with the aid of a stripping gas such as, for example, carbondioxide and/or ammonia, and with addition of heat. It is also possibleto apply thermal stripping. Thermal stripping means that use is madeexclusively of the supply of heat to decompose ammonium carbamate andremove the ammonia and carbon dioxide present from the urea synthesissolution. The gas stream containing ammonia and carbon dioxide thatleaves the stripper is condensed in a high-pressure condenser and thenreturned to the urea synthesis zone.

[0008] The gas mixture that has not reacted in the urea synthesis zoneof a urea stripping plant is removed from the urea synthesis zone andabsorbed at synthesis pressure, for example in a high-pressure scrubber.In such a high-pressure scrubber the condensable components, ammonia andcarbon dioxide, are preferably absorbed from the reactor off-gas into alow-pressure carbamate stream formed in the further recovery. Thecarbamate stream from the high-pressure scrubber, which contains theammonia and carbon dioxide absorbed from the reactor off-gas, isreturned to the urea synthesis zone, optionally via the high-pressurecarbamate condenser. The reactor, high-pressure scrubber, stripper andhigh-pressure condenser are the most important elements of thehigh-pressure section of a urea stripping plant.

[0009] In a urea stripping plant the synthesis reactor is operated at atemperature of 160-240° C. and preferably at a temperature of 170-220°C. The pressure in the synthesis reactor is 12-21 MPa, preferably12.5-19 MPa. The N/C ratio in the urea synthesis zone of a strippingplant lies between 2.5 and 5. The synthesis can be carried out in asingle reactor or in a plurality of reactors arranged in parallel or inseries. When use is made of two reactors in parallel, for example, thefirst reactor can be operated using virtually fresh raw materials andthe second using raw materials entirely or partly recycled, for examplefrom the urea recovery.

[0010] A frequently used embodiment for the preparation of ureaaccording to a stripping process is the Stamicarbon CO₂ strippingprocess as for example described in European Chemical News, UreaSupplement, of Jan. 17, 1969, pages 17-20. The high-pressure condenserin a Stamicarbon CO₂ stripping process is preferably designed as asubmerged high-pressure condenser a so called poolcondensor, asdescribed in NL-A-8400839.

[0011] After the stripping treatment, the pressure of the stripped ureasynthesis solution is reduced in the urea recovery and the solution isevaporated, after which urea is recovered. This produces a low-pressurecarbamate stream in the recovery. This low-pressure carbamate stream ispreferably returned via the high-pressure scrubber to the urea synthesiszone operating at synthesis pressure.

[0012] In a particular embodiment of a urea stripping process thefunctions of reactor and poolcondenser are combined in a singlehigh-pressure vessel with the functionalities of these process stepsbeing separated by partition walls designed for small pressuredifferences in this high-pressure vessel. An example of such anembodiment is described in Nitrogen No. 222, July-August 1996, pages29-31, which describes the poolreactor, as does U.S. Pat. No. 5,767,313.This poolreactor is placed in a horizontal position.

[0013] The disadvantage of this horizontal position is that thehorizontally placed poolreactor takes up a great deal of space and mustalso be placed at a greater height in order to enable the urea synthesissolution to be transferred to the stripper by gravity. This necessitateshigh investments.

[0014] The aim of the present invention now is to provide aninstallation comprising an improved reactor for the preparation of ureawhich requires lower investment costs. The aim of the present inventionis also to provide an improved process for the preparation of urea in aninstallation comprising this reactor.

[0015] The applicant has found an improved installation for thepreparation of urea from ammonia and carbon dioxide, which ischaracterized in that the installation comprises two reactor sections ina vertically placed combined reactor and a high-pressure condensersection.

[0016] In particular, the installation in a first embodiment comprises avertically placed combined reactor comprising two reactor sections thatare separated by a high-pressure condenser section. In a secondembodiment the installation comprises a vertically placed combinedreactor comprising two reactor sections and a high-pressure condensersection placed outside the combined reactor.

[0017] More in particular the applicant has found improved installationsin which in the first embodiment the vertically placed combined reactorconsists of a first reactor section in which a scrubber is present, ahigh-pressure condenser section and a second reactor section, with thehigh-pressure condenser section being located below the first reactorsection in which the scrubber is placed and above the second reactorsection. Preferably, the high-pressure condenser section in the combinedreactor is designed as a submerged high-pressure condenser. In thesecond embodiment the vertically placed reactor comprises two reactorsections, with the second reactor section being located below the firstreactor section, in which the scrubber is installed. The high-pressurecondenser section is placed outside the combined reactor in the secondembodiment of the installation. Preferably, this high-pressure condensersection placed outside the combined reactor is located below thescrubber of the first reactor section, so that the transfer of thecarbamate from the scrubber to the high-pressure condenser takes placeby gravity. Preferably, the carbamate is transferred from the scrubberto the high-pressure condenser through a downcomer. Preferably, thehigh-pressure condenser section placed outside the combined reactor is ahorizontally placed high-pressure condenser and more in particular asubmerged high-pressure condenser as described in EP-A-1 55 735.

[0018] The applicant has also found an improved process for thepreparation of urea from ammonia and carbon dioxide, which ischaracterized in that the preparation takes place wholly or partly in aninstallation comprising two reactor sections in a vertically placedcombined reactor and a high-pressure condenser section. In particular,the applicant has found an improved process for the preparation of ureafrom ammonia and carbon dioxide in which the preparation takes placewholly or partly in an installation in which the vertically placedreactor comprises two reactor sections that are separated by ahigh-pressure condenser section. In another embodiment the preparationof urea from ammonia and carbon dioxide takes place wholly or partly inan installation in which the vertically placed combined reactorcomprises two reactor sections and in which the high-pressure condensersection is placed outside the combined reactor.

[0019] The process for the preparation of urea from ammonia and carbondioxide is characterized in particular in that the gas stream leavingthe stripper is fed to the high-pressure condenser section of theinstallation. More in particular this gas stream is wholly or partlycondensed in the carbamate stream which is transferred from the scrubbersection to the high-pressure condenser section through a downcomer.

[0020] The vertically placed combined reactors in the installation aregenerally designed as a wide pipe with a diameter between 1 and 5metres, preferably between 2 and 4 m. The length of the combined reactoris in general between 5 and 70 metres, preferably between 10 and 40metres.

[0021] The pressure conditions in the reactor, scrubber andhigh-pressure condenser sections of the installation are virtually equaland are such that the reactor sections and the high-pressure condensersection are operated at a high pressure. Preferably, the pressure liesbetween 12 and 22 MPa, in particular between 13 and 21 MPa. Thetemperature in the reactor sections and in the high-pressure condensersections lies between 150 and 250° C., preferably between 170 and 200°C.

[0022] The reactor sections of the vertically placed combined reactor inthe installation are in general provided with means that ensure that thesynthesis solution preferably flows through the reactor sections as aplug flow. For this purpose the reactor sections are provided with forexample a structured packing (in one or more locations) or they aredivided, for example with the aid of sieve plates, into compartments ofvirtually equal volume, so that a cascade-type reactor is formed andtherefore plug flow is approached. The sieve plates used can be of anytype as described in the literature on urea production. The compartmentsform a succession of “continuously stirred tank reactors” (CSTRs), as itwere.

[0023] The number of compartments in the reactor sections of thecombined reactor, as series-arranged CSTRs, is preferably larger than 2and in particular larger than 5. In general, the number of compartments,as CSTRs, will be smaller than 40 and preferably smaller than 20.

[0024] The compartments in the reactor sections of the combined reactorare preferably formed by virtually horizontally placed sieve plates.These preferably have a surface area that is at least 50% of the surfacearea of the horizontal cross-section of the vertically placed reactorand preferably at least 85%. In particular, the sieve plates have asurface area that is virtually equal to 100% of the horizontalcross-section of the vertically placed reactor.

[0025] The heat released in the high-pressure condenser section of theinstallation can be removed by means of water that is passed through oraround the tubes of a heat exchanger, in which process it is convertedinto low-pressure steam of for example 0.3-1 MPa. The heat can beremoved also by passing through a process stream that is to be heated,for example a urea solution to be evaporated. The heat exchanger ispreferably installed in the high-pressure condenser section of theinstallation. If it is placed between the two reactor sections of thevertically placed combined reactor, this high-pressure condenser sectiontakes up 10-70% of the total length of the combined reactor, andpreferably 20-50%.

[0026] The stripping gases can be distributed in the installation forexample by means of a distribution bubble cap in the bottom of thehigh-pressure condenser and they can be wholly or partially condensed inthe carbamate stream coming from the scrubber section through adowncomer. In this process (part of) the gas mixture to be condensedcoming from the stripper is introduced for example into a shell-and-tubeheat exchanger.

[0027] The gas/liquid mixture formed in the high-pressure condensersection of the installation then flows through the tubes of thehigh-pressure condenser, where an exothermic carbamate reaction takesplace. By designing this high-pressure condenser as a submergedcondenser, residence time of the liquid carbamate in the high-pressurecondenser is also ensured, so that urea formation partly takes placealready here.

[0028] In the installation the carbamate stream coming from thehigh-pressure condenser flows together with the urea already formed andwater to the first reactor section of the vertically placed combinedreactor. In this reactor section part of the endothermic urea reactiontakes place.

[0029] The urea solution from the first reactor section is discharged tothe second reactor section. This takes place preferably by making use ofgravity, for instance via a downcomer. This downcomer can be installedboth inside and outside the combined reactor. The transfer of the ureasolution from the first to the second reactor section can also becarried out with an ejector driven by the ammonia required for theprocess.

[0030] In the second reactor section the urea reaction is completed. Ifnecessary for process operation purposes a small portion of the freshcarbon dioxide is fed to the second reactor section.

[0031] As an alternative to the fresh carbon dioxide that is added tothe second reactor section, a portion of the stripping gases can also beused. Preferably, 5-50% of the stripping gas is passed to the secondreactor section using an ammonia-driven ejector, and more in particular10-30% of the stripping gas is passed to the second reactor sectionusing an ammonia-driven ejector. The ammonia needed to drive the ejectorcan be used both in liquid form and in vapour form. The other strippinggas in the installation is preferably passed to the first reactorsection of the vertically placed combined reactor via the high-pressurecondenser section. Use can of course also be made of a combination ofboth fresh carbon dioxide and stripping gases from the stripper to allowthe exothermic carbamate reaction in the second reactor section toproceed.

[0032] The reactor off-gases with still free ammonia and carbon dioxideare washed in the scrubber section of the installation with thelow-pressure carbamate stream that is formed in the further recoveryand/or the ammonia feed. Preferably, the fresh ammonia feed is usedwholly or partially as an absorbent in the scrubber section of theinstallation. In this scrubber total as well as partial washing of thisunconverted ammonia and carbon dioxide can take place. If necessary, thereactor off-gases can be freed of remaining ammonia and carbon dioxideoutside the combined reactor.

[0033] The conversion of carbamate into urea and water in theinstallation can be accomplished by ensuring a sufficiently longresidence time of the reaction mixture in the vertically placed combinedreactor. The residence time will in general be more than 10 min.,preferably more than 20 min. The residence time will in general beshorter than 2 hours, preferably shorter than 1 hour. At a highertemperature and pressure in the combined reactor a short residence timeis usually sufficient to obtain a high conversion.

[0034] The installation according to the present invention can beapplied in new plants (grassroots plants) as well as for the improvementand optimization (revamping) of existing urea plants of any design.

[0035] The invention therefore also relates to a method for improvingand optimizing (revamping) of existing urea plants by installing aninstallation according to the present invention. In particular, theinvention relates to a method for improving and optimizing existing ureaplants by replacing the existing reactor and high-pressure condenserwith an installation according to the invention. Such replacement can bedone in conventional plants as well as in stripping plants of anydesign.

[0036] Since the installation comprises a vertically placed reactor,this combined reactor needs only a limited floor area, which offers theexceptional advantage that the combined reactor can be installed atground level, while the urea solution is discharged by gravity to thehigh-pressure stripper. Especially in revamping projects the availablefloor area is often limited and therefore the vertically placed combinedreactor is eminently suitable for this. The vertically placed combinedreactor is also an attractive alternative to, for example, thepoolreactor.

[0037] The added advantage of the installation according to theinvention in revamping of conventional plants is the fact that the steamconsumption is comparable with the steam consumption in strippingplants, i.e. about 925 kg steam per tonne of urea. For a conventionalurea plant this is a remarkable improvement.

[0038] A great advantage of the installation comprising the combinedreactor comprising two reactor sections that are separated by ahigh-pressure condenser section is that it can be introduced into aplant with substantially lower investment costs, because due to theintegration of a heat exchanger/high-pressure condenser and scrubberinto a combined reactor fewer equipment items and lines—that must beresistant to high pressure in a very corrosive environment—arenecessary. A further advantage is the installation at ground level,resulting in a less high plant structure. Installation at ground levelis also possible for the second embodiment of the installation, in whichthe combined reactor comprises two reactor sections and in which thehigh-pressure condenser section is placed outside the combined reactor.The installation at ground level offers further advantages in terms ofinvestment and also promotes safety.

[0039] The invention also relates to a urea plant in which thehigh-pressure section substantially consists of an installationaccording to the invention, comprising two reactor sections in avertically placed combined reactor, a high-pressure condenser sectionand a high-pressure stripper. In particular, the invention relates to aurea plant in which the high-pressure section substantially consists ofan installation according to the invention, the vertically placedcombined reactor comprising two reactor sections that are separated by ahigh-pressure condenser section and in which also a high-pressurestripper is installed. The invention also relates to a urea plant inwhich the high-pressure section substantially consists of aninstallation comprising a vertically placed combined reactor comprisingtwo reactor sections, a high-pressure condenser installed outside thereactor and a high-pressure stripper. The stripper used in thehigh-pressure section of a urea plant is preferably a CO₂ stripper. Inparticular, the invention relates to a urea plant in which thehigh-pressure section can be placed at ground level and in which theurea solution is gravity-fed to the stripper.

[0040] By way of example the invention is further elucidated below onthe basis of the following figures and examples.

[0041]FIG. 1: A schematic representation of part of a urea strippingplant according to the Stamicarbon CO₂ stripping process.

[0042]FIG. 2: A schematic representation of part of a new urea strippingplant according to the principle of the invention, the installationcomprising a vertically placed combined reactor comprising two reactorsections that are separated by a high-pressure condenser section.

[0043]FIG. 3: A schematic representation of part of a new urea strippingplant according to the principle of the invention, the installationcomprising a vertically placed reactor comprising two reactor sectionsthat are separated by a high-pressure condenser section, with an ammoniaejector.

[0044]FIG. 4A: A schematic representation of part of a urea strippingplant according to the principle of the invention, the installationcomprising a vertically placed combined reactor comprising two reactorsections that are separated by a high-pressure condenser section, with astripping gas ejector.

[0045]FIG. 4B: A schematic representation of a urea stripping plantaccording to the principle of the invention, the installation comprisinga vertically placed combined reactor comprising two reactor sections anda high-pressure condenser section placed outside the combined reactor,with a stripping gas ejector.

[0046]FIG. 5A: A schematic representation of part of a combined reactor,comprising two reactor sections that are separated by a high-pressurecondenser section.

[0047]FIG. 5B: A schematic representation of part of a combined reactor,comprising two reactor sections.

[0048] In FIG. 1, R represents a reactor in a Stamicarbon CO₂ strippingplant in which carbon dioxide and ammonia are converted into urea. Theurea synthesis solution (USS) leaving the reactor is sent to a CO₂stripper (S), in which the USS is converted into a gas stream (SG) and aliquid stream (SUSS) by stripping the USS with CO₂. The gas stream (SG)leaving the CO₂ stripper substantially consists of ammonia and carbondioxide and the SUSS is the stripped USS. The stream containing thestripped urea synthesis solution (SUSS) is passed to the urea recovery(UR), where urea (U) is recovered and water (W) is discharged. In the URa low-pressure ammonium carbamate stream (LPC) is obtained, which is fedto the high-pressure scrubber (SCR). In this scrubber the LPC iscontacted with the gas stream coming from the reactor (RG), whichsubstantially consists of ammonia and carbon dioxide but which alsocontains the inerts (non-condensable components) present in the carbondioxide feed and the ammonia feed. Normally, heat is also carried off inthe SCR. In this example, the enriched carbamate stream (EC) leaving theSCR is passed to the high-pressure condenser (C), in which the SG streamis condensed with the aid of EC. This condensation may also be effectedwithout adding EC to C; in this case it is logical for EC to be added tothe reactor R direct. The resulting high-pressure carbamate stream (HPC)is returned to the reactor. In this example, the fresh ammonia isrecycled via the high-pressure carbamate condenser (C) but it may ofcourse also be admitted elsewhere in the R->S->C->R loop or in theR->SCR->C->R loop.

[0049] In FIG. 2 ECR represents a combined reactor comprising tworeactor sections that are separated by a high-pressure condenser sectionin a CO₂ stripping plant in which carbon dioxide and ammonia areconverted into urea. The urea solution (UCS) coming from the firstsection of the reactor is supplied to the second section of the reactorin which the urea reaction is completed. The urea solution (USS) comingfrom the second section of the reactor is transferred to a CO₂ stripper(S), in which the USS is converted into a gas stream (SG) and a liquidstream (SUSS) by stripping the USS with carbon dioxide. The gas streamleaving the stripper (SG) substantially consists of ammonia and carbondioxide and the SUSS is the stripped USS. The stream containing thestripped urea synthesis solution SUSS is transferred to the urearecovery (UR), where urea (UR) is recovered and water (W) is discharged.In the UR a low-pressure ammonium carbamate stream (LPC) is obtained,which is fed to a scrubber bed in the combined reactor. In this scrubberthe LPC is contacted with the gas stream (CRG) coming from the firstsection of the combined reactor, which substantially consists of ammoniaand carbon dioxide, but which in addition contains the inerts(non-condensable components) present in the carbon dioxide feed and theammonia feed. Also, in this figure the ammonia feed is by way of exampleused as absorbent in this scrubber. The enriched carbamate stream (EC)coming from this scrubber is transferred to the high-pressure condensersection of the combined reactor. Via a downcomer, which may be situatedoutside the reactor but is preferably situated within the reactor, thiscarbamate stream (of high pressure) is contacted with the ammonia andcarbon dioxide vapour in the high-pressure condenser section of thecombined reactor, which is of the submerged type with limited residencetime. The carbamate and the urea formed in this high-pressure condensersection are transferred to the first reaction section of the combinedreactor, in which the urea reaction takes place to a substantial degree.The urea solution, which also contains unconverted carbamate (UCS), isgravitated to the second reaction section of the combined reactor. Forstirring of the compartments in this reactor section and for completionof the urea reaction, fresh CO₂ is fed to this second reactor section.The gas stream (RG) leaving the second reactor section, which streamsubstantially consists of ammonia and carbon dioxide but in additioncontains the inerts (non-condensable components) present in the carbondioxide feed, is fed to the high-pressure condenser section of thecombined reactor. The separation of the gas stream leaving the secondreactor section from the urea synthesis solution (USS) leaving thisreactor section preferably takes place in the combined reactor, but itcan also take place in a gas/liquid separator installed for the purposethat is situated outside the combined reactor.

[0050]FIG. 3 is a diagrammatic representation of part of a urea plant asin FIG. 2 in which the urea solution (UCS), which also containsunconverted carbamate, is transferred to the second reaction section ofthe combined reactor according to the first embodiment by means of anammonia-driven ejector.

[0051]FIG. 4A is a diagrammatic representation of a urea stripping plantas in FIG. 2 in which an ejector driven by the required ammonia is usedto transfer a portion of the stripped gas (SG) to the second reactorsection for stirring of the compartments in this second reactor sectionand for completion of the urea reaction.

[0052]FIG. 4B is a diagrammatic representation of a urea plant as inFIG. 4A in which the vertical placed combined reactor comprising tworeactor sections that are separated by a high-pressure condenser sectionhas been replaced by a vertically placed combined reactor comprising tworeactor sections (ECRR) and a high-pressure condenser section placedoutside the combined reactor (HPCC). The high-pressure condenser sectionplaced outside the combined reactor is, according to a preferredembodiment of the invention, shown as a submerged high-pressurecondenser.

[0053]FIG. 5A is a diagrammatic representation of part of the verticallyplaced combined reactor, comprising two reactor sections that areseparated by a high-pressure condenser section, that is used in FIGS.2,3 and 4A. In this figure the symbols denote the following:

[0054] R1 first reactor section

[0055] R2 second reactor section

[0056] C high-pressure condenser section

[0057] SCR scrubber section

[0058] B1 overflow of the first reactor section

[0059] B2 overflow of the second reactor section

[0060] B3 urea synthesis solution feed to second reactor section

[0061] B4 stripper gas feed to high-pressure condenser section

[0062] B5 manhole

[0063] B6 CO₂ feed to second reactor section

[0064] B7 steam/condensate discharge of high-pressure condenser section

[0065] B8 condensate feed to high-pressure condenser section

[0066] B9 radioactive level measurement

[0067] B10 low-pressure carbamate/ammonia feed

[0068] B11 reactor off-gas discharge

[0069] B12 blow-off/safety valve

[0070] C1 thermocouples

[0071] C2 thermocouples

[0072]FIG. 5B is a diagrammatic representation of the vertically placedcombined reactor used in FIG. 4B. The codes used are the same as in FIG.5A. Further, in FIG. 5B: DP is a downcomer through which the enrichedcarbamate stream leaving the scrubber is transferred to thehigh-pressure condenser B13 is the stripper gas feed to the secondreactor section B14 is the high-pressure carbamate feed from thecondenser.

[0073] The invention is elucidated further by means of the followingexamples.

COMPARATIVE EXAMPLE A

[0074] Table 1 below presents the compositions in percent by weight ofthe various streams for a Stamicarbon CO₂ stripping plant as shown inFIG. 1. From the compositions it follows that the urea conversion hastaken place almost entirely in the reactor (R) and the carbamatecondensation has taken place in the high-pressure condenser (C). TABLE 1Stream Urea NH₃ CO₂ H₂O Inerts USS 33.0 30.5 18.0 18.5 — CO₂ — — 96.0 0.5  3.5 SUSS 55.0  7.8 10.5 26.7 — SG — 41.0 54.5  3.5  1.0 NH₃ — 99.6—  0.4 — HPC — 49.5 42.0  8.0  0.5 RG — 50.0 39.5  3.5  7.0 EC — 39.039.0 22.0 — LPC — 30.0 37.0 33.0 — Inerts —  5.5  5.0  0.5 89.0

EXAMPLE 1

[0075] Table 2 below presents the compositions in percent by weight ofthe various streams for a Stamicarbon CO₂ stripping plant as shown inFIG. 2, in which a combined reactor comprising two reactor sections thatare separated by a high-pressure section is installed. A substantialpart of the urea reaction takes place in the first section of thecombined reactor and the urea reaction is completed in the secondreactor section of the combined reactor. TABLE 2 Stream Urea NH₃ CO₂ H₂OInerts USS 35.5 29.5 16.5 18.5 — CO₂ — — 96.0  0.5  3.5 SUSS 55.0  7.810.5 26.7 — SG — 40.0 54.0  3.5  2.5 NH₃ — 99.6 —  0.4 — UCS 23.5 34.526.0 16.0 — CRG — 56.0 37.0  2.5  4.5 RG — 39.0 39.0 22.0 — LPC — 30.037.0 33.0 — Inerts —  5.5  5.0  0.5 89.0

1. Installation for the preparation of urea from ammonia and carbondioxide, characterized in that the installation comprises two reactorsections in a vertically placed combined reactor and a high-pressurecondenser section.
 2. Installation according to claim 1, characterizedin that the installation comprises a vertically placed combined reactorcomprising two reactor sections that are separated by a high-pressurecondenser section.
 3. Installation according to claim 1, characterizedin that the installation comprises a vertically placed combined reactorcomprising two reactor sections and a high-pressure condenser sectionplaced outside the combined reactor.
 4. Installation according to claim2, characterized in that in the combined reactor the high-pressurecondenser section is located below the reactor section in which thescrubber is placed and above the second reactor section.
 5. Installationaccording to claim 3, characterized in that in the combined reactor thereactor section in which the scrubber is located is placed above thesecond reactor section.
 6. Installation according to claim 3,characterized in that the high-pressure condenser section placed outsidethe reactor is located below the scrubber of the upper reactor sectionof the combined reactor.
 7. Installation according to any one of claims1-6, characterized in that the reactor sections in the combined reactorare provided with means that ensure that the synthesis solutionsubstantially flows through the reactor as a plug flow.
 8. Installationaccording to any one of claims 1-6, characterized in that the number ofcompartments in the reactor sections of the combined reactor, in theform of series-arranged CSTRs, is greater than
 2. 9. Installationaccording to any one of claims 3, 5-6, characterized in that thehigh-pressure condenser section is designed as a horizontally placedsubmerged condenser.
 10. Installation according to any one of claims1-8, characterized in that the high-pressure condenser section isdesigned as a submerged high-pressure condenser.
 11. Process for thepreparation of urea from ammonia and carbon dioxide, characterized inthat the preparation takes place wholly or partly in an installationaccording to any one of claims 1-10.
 12. Process according to claim 11,characterized in that the gas stream leaving the stripper is fed whollyor partly to the high-pressure condenser section of an installationaccording to any one of claims 1-10.
 13. Process according to either ofclaims 11-12, characterized in that the gas stream leaving the stripperis wholly or partly condensed in the carbamate stream that istransferred from the scrubber to the high-pressure condenser sectionthrough a downcomer.
 14. Process according to claim 11, characterized inthat a portion of the gas stream leaving the stripper is fed to thesecond reactor section in the vertically placed combined reactor via anammonia-driven ejector.
 15. Process according to claim 14, characterizedin that 5-50% of the gas stream leaving the stripper is fed to thesecond reactor section in the vertically placed combined reactor via anammonia-driven ejector.
 16. Process according to claim 15, characterizedin that 10-30% of the gas stream leaving the stripper is fed to thesecond reactor section in the vertically placed combined reactor via anammonia-driven ejector.
 17. Process according to any one of claims14-16, characterized in that the remainder of the gas stream from thestripper is passed via the high-pressure condenser to the first reactorsection, where the scrubber is located.
 18. Process according to any oneof claims 11-17, characterized in that the ammonia feed is wholly orpartially used as absorbent in the scrubber section of the combined ureareactor.
 19. Method for improving and optimizing an existing urea plant,characterized in that an installation according to any one of claims1-10 is installed.
 20. Method according to claim 19, characterized inthat the existing reactor and high-pressure condenser are replaced by aninstallation according to any one of claims 1-10.
 21. Urea plant,characterized in that the high-pressure section substantially consistsof an installation according to any one of claims 1-10 and ahigh-pressure stripper.